Protein kinase C (PKC) comprises a family of several related isoenzymes that function as serine/threonine kinases. PKC plays an important role in intercellular and intracellular signaling, gene expression, and in the control of cell differentiation and growth. Currently, at least ten isoforms of PKC are known which are different in regulation, tissue distribution, and enzymatic specificity (Newton A C. Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm. Biochem J 2003; 370(Pt 2):361-371; Newton A C. Protein kinase C: poised to signal. Am J Physiol Endocrinol Metab 2010; 298(3):E395-E402; Nishizuka Y. Studies and prospectives of the protein kinase c family for cellular regulation. Cancer 1989; 63(10):1892-1903; Nishizuka Y. The Albert Lasker Medical Awards. The family of protein kinase C for signal transduction. JAMA 1989; 262(13):1826-1833). The PKC family of isoenzymes are grouped into three subclasses based on the domain composition of the regulatory moiety: (1) conventional PKCs (alpha, beta-II, and beta-I), (2) novel PKCs (delta, epsilon, gamma, eta and theta) and (3) atypical PKCs (zeta and iota/lambda) (Newton A C. Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm. Biochem J 2003; 370(Pt 2):361-371; Mellor H, Parker P J. The extended protein kinase C superfamily. Biochem J 1998; 332 (Pt 2):281-292). PKC is a membrane-associated enzyme that is regulated by several distinct factors, such as membrane phospholipids, calcium, and membrane lipids, e.g. diacylglycerol (Newton A C. Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm. Biochem J 2003; 370(Pt 2):361-371; Newton A C. Protein kinase C: poised to signal. Am J Physiol Endocrinol Metab 2010; 298(3):E395-E402; Mellor H, Parker P J. The extended protein kinase C superfamily. Biochem J 1998; 332 (Pt 2):281-292; Kishimoto A, Kikkawa U, Ogita K, Shearman M S, Nishizuka Y. The protein kinase C family in the brain: heterogeneity and its implications. Ann N Y Acad Sci 1989; 568: 181-186; Nishizuka Y. Calcium, phospholipid turnover and transmembrane signalling. Philos Trans R Soc Lond B Biol Sci 1983; 302(1108):101-112.). All PKC isoforms have an autoinhibitory pseudosubstrate sequence that is N-terminal to the C1 domain, which functions as a diacylglycerol sensor. Atypical PKCs have a diacylglycerol non-responsive C1 domain. Conventional PKCs have a C2 domain that serves as a Ca2+-regulated phospholipid-binding module. The C2 domain in novel PKCs binds neither Ca2+ nor membrane phospholipids. Based on the structural differences conventional PKCs require membrane phospholipids, calcium and diacylglycerol for complete activation. Novel PKCs do not require calcium but diacylglycerol for activation. The zeta and iota/lambda forms of PKC are independent of both calcium and diacylglycerol for their activation (Newton A C. Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm. Biochem J 2003; 370(Pt 2):361-371; Newton A C. Lipid activation of protein kinases. J Lipid Res 2009; 50 Suppl:S266-S271). PKC is involved in the regulation of smooth muscle contractility. Upon stimulation PKC phosphorylates the regulatory myosin light chain (MLC20) and inhibits the myosin associated phosphatase (MYPT). Phosphorylation of MLC20 and inhibition of MYPT leads to an increased activity of the acto-myosin complex and to vasoconstriction in different vascular beds, e.g. resistance-sized, retinal, cerebral, coronary, conduit arteries and veins (Merkel L A, Rivera L M, Colussi D J, Perrone M H. Protein kinase C and vascular smooth muscle contractility: effects of inhibitors and down-regulation. J Pharmacol Exp Ther 1991; 257(1):134-140; Sehic E, Malik K U. Influence of protein kinase C activators on vascular tone and adrenergic neuroeffector events in the isolated rat kidney. J Pharmacol Exp Ther 1989; 251(2):634-639.).
Overexpressed or overactivated PKC detrimentally affects heart function. Upon activation PKC affects the intracellular calcium homeostasis which results in reduced myocardial contractility and relaxation of the myocardium. Overall this effect leads to myocardial contractile insufficiency (Connelly K A, Kelly D J, Zhang Y, Prior D L, Advani A, Cox A J, That K, Krum H, Gilbert R E. Inhibition of protein kinase C-beta by ruboxistaurin preserves cardiac function and reduces extracellular matrix production in diabetic cardiomyopathy. Circ Heart Fail 2009; 2(2):129-137). Moreover, activated PKC mediates organ damage during end-organ injuries, e.g. during ischemia in heart (Connelly K A, Kelly D J, Zhang Y, Prior D L, Advani A, Cox A J, That K, Krum H, Gilbert R E. Inhibition of protein kinase C-beta by ruboxistaurin preserves cardiac function and reduces extracellular matrix production in diabetic cardiomyopathy. Circ Heart Fail 2009; 2(2):129-137; Hambleton M, Hahn H, Pleger S T, Kuhn M C, Klevitsky R, Carr A N, Kimball T F, Hewett T E, Dorn G W, Koch W J, Molkentin J D. Pharmacological—and gene therapy-based inhibition of protein kinase Calpha/beta enhances cardiac contractility and attenuates heart failure. Circulation 2006; 114(6): 574-582) or kidney (Tuttle K R. Protein kinase C-beta inhibition for diabetic kidney disease. Diabetes Res Clin Pract 2008; 82 Suppl 1:S70-S74; Anderson P W, McGill J B, Tuttle K R. Protein kinase C beta inhibition: the promise for treatment of diabetic nephropathy. Curr Opin Nephrol Hypertens 2007; 16(5):397-402). PKC and especially the PKC-beta II isoform is overexpressed or overactivated in diabetes in various different types of tissue and exerts its deleterious effect to the cells, tissues and end-organs, e.g. kidney (Tuttle K R. Protein kinase C-beta inhibition for diabetic kidney disease. Diabetes Res Clin Pract 2008; 82 Suppl 1:S70-S74; Anderson P W, McGill J B, Tuttle K R. Protein kinase C beta inhibition: the promise for treatment of diabetic nephropathy. Curr Opin Nephrol Hypertens 2007; 16(5):397-402; Tuttle K R, Bakris G L, Toto R D, McGill J B, Hu K, Anderson P W. The effect of ruboxistaurin on nephropathy in type 2 diabetes. Diabetes Care 2005; 28(11):2686-2690; Kelly D J, Zhang Y, Hepper C, Gow R M, Jaworski K, Kemp B E, Wilkinson-Berka J L, Gilbert R E. Protein kinase C beta inhibition attenuates the progression of experimental diabetic nephropathy in the presence of continued hypertension. Diabetes 2003; 52(2):512-518), heart (Connelly K A, Kelly D J, Zhang Y, Prior D L, Advani A, Cox A J, That K, Krum H, Gilbert R E. Inhibition of protein kinase C-beta by ruboxistaurin preserves cardiac function and reduces extracellular matrix production in diabetic cardiomyopathy. Circ Heart Fail 2009; 2(2):129-137; Guo M, Wu M H, Korompai F, Yuan S Y. Upregulation of PKC genes and isozymes in cardiovascular tissues during early stages of experimental diabetes. Physiol Genomics 2003; 12(2):139-146), or in tissues like the retina (Aiello L P, Clermont A, Arora V, Davis M D, Sheetz M J, Bursell S E. Inhibition of PKC beta by oral administration of ruboxistaurin is well tolerated and ameliorates diabetes-induced retinal hemodynamic abnormalities in patients. Invest Ophthalmol V is Sci 2006; 47(1):86-92; Aiello L P. The potential role of PKC beta in diabetic retinopathy and macular edema. Surv Ophthalmol 2002; 47 Suppl 2:S263-S269; Kimura M, Ishizawa M, Miura A, Itaya S, Kanoh Y, Yasuda K, Uno Y, Morita H, Ishizuka T. Platelet protein kinase C isoform content in type 2 diabetes complicated with retinopathy and nephropathy. Platelets 2001; 12(3):138-143) or neuronal tissue (Krishnan S T, Rayman G. New treatments for diabetic neuropathy: symptomatic treatments. Curr Diab Rep 2003; 3(6):459-467; Kim H, Sasaki T, Maeda K, Koya D, Kashiwagi A, Yasuda H. Protein kinase Cbeta selective inhibitor LY333531 attenuates diabetic hyperalgesia through ameliorating cGMP level of dorsal root ganglion neurons. Diabetes 2003; 52(8):2102-2109; Cotter M A, Jack A M, Cameron N E. Effects of the protein kinase C beta inhibitor LY333531 on neural and vascular function in rats with streptozotocin-induced diabetes. Clin Sci (Lond) 2002; 103(3):311-321; Nakamura J, Kato K, Hamada Y, Nakayama M, Chaya S, Nakashima E, Naruse K, Kasuya Y, Mizubayashi R, Miwa K, Yasuda Y, Kamiya H, Ienaga K, Sakakibara F, Koh N, Hotta N. A protein kinase C-beta-selective inhibitor ameliorates neural dysfunction in streptozotocin-induced diabetic rats. Diabetes 1999; 48(10):2090-2095) or in platelets (Assert R, Scherk G, Bumbure A, Pirags V, Schatz H, Pfeiffer A F. Regulation of protein kinase C by short term hyperglycaemia in human platelets in vivo and in vitro. Diabetologia 2001; 44(2):188-195; Bynagari-Settipalli Y S, Chari R, Kilpatrick L, Kunapuli S P. Protein kinase C—possible therapeutic target to treat cardiovascular diseases. Cardiovasc Hematol Disord Drug Targets 2010; 10(4):292-308; Kimura M, Ishizawa M, Miura A, Itaya S, Kanoh Y, Yasuda K, Uno Y, Morita H, Ishizuka T. Platelet protein kinase C isoform content in type 2 diabetes complicated with retinopathy and nephropathy. Platelets 2001; 12(3):138-143; Oskarsson H J, Hofineyer T G, Coppey L, Yorek M A. Effect of protein kinase C and phospholipase A2 inhibitors on the impaired ability of human diabetic platelets to cause vasodilation. Br J Pharmacol 1999; 127(4):903-908) or induces endothelial dysfunction (Chiasson V L, Quinn M A, Young K J, Mitchell B M. Protein kinase Cbetall-mediated phosphorylation of endothelial nitric oxide synthase threonine 495 mediates the endothelial dysfunction induced by FK506 (tacrolimus). J Pharmacol Exp Ther 2011; 337(3):718-723; Xu Y, Wang S, Feng L, Zhu Q, Xiang P, He B. Blockade of PKC-beta protects HUVEC from advanced glycation end products induced inflammation. Int Immunopharmacol 2010; 10(12):1552-1559; Geraldes P, King G L. Activation of protein kinase C isoforms and its impact on diabetic complications. Circ Res 2010; 106(8):1319-1331; Nacci C, Tarquinio M, Montagnani M. Molecular and clinical aspects of endothelial dysfunction in diabetes. Intern Emerg Med 2009; 4(2):107-116). Furthermore, it has been suggested that PKC signalling is involved in tumour formation (Gonelli A, Mischiati C, Guerrini R, Voltan R, Salvadori S, Zauli G. Perspectives of protein kinase C(PKC) inhibitors as anti-cancer agents. Mini Rev Med Chem 2009; 9(4):498-509; Ali A S, Ali S, El-Rayes B F, Philip P A, Sarkar F H. Exploitation of protein kinase C: a useful target for cancer therapy. Cancer Treat Rev 2009; 35(1):1-8), e.g. in hematological tumours (Mischiati C, Melloni E, Corallini F, Milani D, Bergamini C, Vaccarezza M. Potential role of PKC inhibitors in the treatment of hematological malignancies. Curr Pharm Des 2008; 14(21):2075-2084; Cheson B D, Zwiebel J A, Dancey J, Murgo A. Novel therapeutic agents for the treatment of myelodysplastic syndromes. Semin Oncol 2000; 27(5):560-577; Deng X, Kornblau S M, Ruvolo P P, May W S, Jr. Regulation of Bcl2 phosphorylation and potential significance for leukemic cell chemoresistance. J Natl Cancer Inst Monogr 2001; (28):30-37), in glioma formation (Baltuch G H, Dooley N P, Villemure J G, Yong V W. Protein kinase C and growth regulation of malignant gliomas. Can J Neurol Sci 1995; 22(4):264-271; Blobe G C, Obeid L M, Hannun Y A. Regulation of protein kinase C and role in cancer biology. Cancer Metastasis Rev 1994; 13(3-4):411-431; Bredel M, Pollack I F. The role of protein kinase C(PKC) in the evolution and proliferation of malignant gliomas, and the application of PKC inhibition as a novel approach to anti-glioma therapy. Acta Neurochir (Wien)) 1997; 139(11):1000-1013), in gastric and intestinal cancer (Atten M J, Godoy-Romero E, Attar B M, Milson T, Zopel M, Holian O. Resveratrol regulates cellular PKC alpha and delta to inhibit growth and induce apoptosis in gastric cancer cells. Invest New Drugs 2005; 23(2):111-119; Fahrmann M. Targeting protein kinase C (PKC) in physiology and cancer of the gastric cell system. Curr Med Chem 2008; 15(12):1175-1191), in skin cancer (Birt D F, Yaktine A, Duysen E. Glucocorticoid mediation of dietary energy restriction inhibition of mouse skin carcinogenesis. J Nutr 1999; 129 (2S Suppl):571S-574S; Birt D F, Przybyszewski J, Wang W, Stewart J, Liu Y. Identification of molecular targets for dietary energy restriction prevention of skin carcinogenesis: an idea cultivated by Edward Bresnick. J Cell Biochem 2004; 91(2):258-264), lung cancer (Herbst R S, Oh Y, Wagle A, Lahn M. Enzastaurin, a protein kinase Cbeta-selective inhibitor, and its potential application as an anticancer agent in lung cancer. Clin Cancer Res 2007; 13(15 Pt 2):s4641-s4646; Herbst R S. Targeted therapy in non-small-cell lung cancer. Oncology (Williston Park) 2002; 16(9 Suppl 9):19-24) and others. PKC is an important signal transducer of events in autoimmune responses, e.g. in T-cell (Birchall A M, Bishop J, Bradshaw D, Cline A, Coffey J, Elliott L H, Gibson V M, Greenham A, Hallam T J, Harris W. Ro 32-0432, a selective and orally active inhibitor of protein kinase C prevents T-cell activation. J Pharmacol Exp Ther 1994; 268(2):922-929; Isakov N, Altman A. Protein kinase C(theta) in T cell activation. Annu Rev Immunol 2002; 20:761-794) or B-cell (Shinohara H, Kurosaki T. Comprehending the complex connection between PKCbeta, TAK1, and IKK in BCR signaling. Immunol Rev 2009; 232(1):300-318; Venkataraman C, Chen X C, Na S, Lee L, Neote K, Tan S L. Selective role of PKCbeta enzymatic function in regulating cell survival mediated by B cell antigen receptor cross-linking. Immunol Lett 2006; 105(1):83-89) linked autoimmune signalling, and in inflammatory processes.
The above mentioned effects of the PKC-mediated signalling leads to induction or promotion of the progression of asthma (Boschelli D H. Small molecule inhibitors of PKCTheta as potential antiinflammatory therapeutics. Curr Top Med Chem 2009; 9(7):640-654), chronic obstructive pulmonary disease (Mercer B A, D'Armiento J M. Emerging role of MAP kinase pathways as therapeutic targets in COPD. Int J Chron Obstruct Pulmon Dis 2006; 1(2):137-150; Adcock I M, Chung K F, Caramori G, Ito K. Kinase inhibitors and airway inflammation. Eur J Pharmacol 2006; 533(1-3):118-132; Dempsey E C, Cool C D, Littler C M. Lung disease and PKCs. Pharmacol Res 2007; 55(6):545-559; Ishii M, Kurachi Y. Muscarinic acetylcholine receptors. Curr Pharm Des 2006; 12(28):3573-3581; Medina-Tato D A, Watson M L, Ward S G. Leukocyte navigation mechanisms as targets in airway diseases. Drug Discov Today 2006; 11(19-20):866-879), pulmonary hypertension (Agbani E O, Coats P, Mills A, Wadsworth R M. Peroxynitrite stimulates pulmonary artery endothelial and smooth muscle cell proliferation: involvement of ERK and PKC. Pulm Pharmacol Ther 2011; 24(1):100-109; Littler C M, Wehling C A, Wick M J, Fagan K A, Cool C D, Messing R O, Dempsey E C. Divergent contractile and structural responses of the murine PKC-epsilon null pulmonary circulation to chronic hypoxia. Am J Physiol Lung Cell Mol Physiol 2005; 289(6):L1083-L1093), retinopathy, like retinal ischemia and neovascularization (Galvez M I. Protein kinase C inhibitors in the treatment of diabetic retinopathy. Review. Curr Pharm Biotechnol 2011; 12(3):386-391; Schwartz S G, Flynn H W, Jr., Aiello L P. Ruboxistaurin mesilate hydrate for diabetic retinopathy. Drugs Today (Barc) 2009; 45(4):269-274), nephropathy, including hypertension-induced (Kelly D J, Edgley A J, Zhang Y, That K, Tan S M, Cox A J, Advani A, Connelly K A, Whiteside C I, Gilbert R E. Protein kinase C-beta inhibition attenuates the progression of nephropathy in non-diabetic kidney disease. Nephrol Dial Transplant 2009; 24(6):1782-1790; Hayashi K, Wakino S, Ozawa Y, Homma K, Kanda T, Okubo K, Takamatsu I, Tatematsu S, Kumagai H, Saruta T. Role of protein kinase C in Ca channel blocker-induced renal arteriolar dilation in spontaneously hypertensive rats—studies in the isolated perfused hydronephrotic kidney. Keio J Med 2005; 54(2):102-108; Kelly D J, Zhang Y, Hepper C, Gow R M, Jaworski K, Kemp B E, Wilkinson-Berka J L, Gilbert R E. Protein kinase C beta inhibition attenuates the progression of experimental diabetic nephropathy in the presence of continued hypertension. Diabetes 2003; 52(2):512-518), non-hypertension-induced, and diabetic nephropathies (Danis R P, Sheetz M J. Ruboxistaurin: PKC-beta inhibition for complications of diabetes. Expert Opin Pharmacother 2009; 10(17):2913-2925; Tuttle K R. Protein kinase C-beta inhibition for diabetic kidney disease. Diabetes Res Clin Pract 2008; 82 Suppl 1:S70-S74), renal failure (Danis R P, Sheetz M J. Ruboxistaurin: PKC-beta inhibition for complications of diabetes. Expert Opin Pharmacother 2009; 10(17):2913-2925; Yamagishi S, Fukami K, Ueda S, Okuda S. Molecular mechanisms of diabetic nephropathy and its therapeutic intervention. Curr Drug Targets 2007; 8(8):952-959) and myocardial infarction (Bynagari-Settipalli Y S, Chari R, Kilpatrick L, Kunapuli S P. Protein kinase C—possible therapeutic target to treat cardiovascular diseases. Cardiovasc Hematol Disord Drug Targets 2010; 10(4):292-308; Rohilla A, Singh G, Singh M, Bala kP. Possible involvement of PKC-delta in the abrogated cardioprotective potential of ischemic preconditioning in hyperhomocysteinemic rat hearts. Biomed Pharmacother 2010; 64(3):195-202; Liu Q, Chen X, Macdonnell S M, Kranias E G, Lorenz J N, Leitges M, Houser S R, Molkentin J D. Protein kinase C{alpha}, but not PKC{beta} or PKC {gamma}, regulates contractility and heart failure susceptibility: implications for ruboxistaurin as a novel therapeutic approach. Circ Res 2009; 105(2):194-200; Yonezawa T, Kurata R, Kimura M, Inoko H. PKC delta and epsilon in drug targeting and therapeutics. Recent Pat DNA Gene Seq 2009; 3(2):96-101) cardiac hypertrophy and failure (Ferreira J C, Brum P C, Mochly-Rosen D. betalIPKC and epsilonPKC isozymes as potential pharmacological targets in cardiac hypertrophy and heart failure. J Mol Cell Cardiol 2010; Palaniyandi S S, Sun L, Ferreira J C, Mochly-Rosen D. Protein kinase C in heart failure: a therapeutic target? Cardiovasc Res 2009; 82(2):229-239), coronary heart disease, artherosclerosis, restenosis (Ding R Q, Tsao J, Chai H, Mochly-Rosen D, Zhou W. Therapeutic potential for protein kinase C inhibitor in vascular restenosis. J Cardiovasc Pharmacol Ther 2011; 16(2):160-167; Schleicher E, Friess U. Oxidative stress, AGE, and atherosclerosis. Kidney Int Suppl 2007; (106):517-526), diabetes, diabetic complications, glucose utilization and metabolic syndrome (Bynagari-Settipalli Y S, Chari R, Kilpatrick L, Kunapuli S P. Protein kinase C—possible therapeutic target to treat cardiovascular diseases. Cardiovasc Hematol Disord Drug Targets 2010; 10(4):292-308; Geraldes P, King G L. Activation of protein kinase C isoforms and its impact on diabetic complications. Circ Res 2010; 106(8):1319-1331; Danis R P, Sheetz MJ. Ruboxistaurin: PKC-beta inhibition for complications of diabetes. Expert Opin Pharmacother 2009; 10(17):2913-2925), immune diseases (Baier G, Wagner J. PKC inhibitors: potential in T cell-dependent immune diseases. Curr Opin Cell Biol 2009; 21(2):262-267; Mecklenbrauker I, Saijo K, Zheng N Y, Leitges M, Tarakhovsky A. Protein kinase Cdelta controls self-antigen-induced B-cell tolerance. Nature 2002; 416(6883):860-865; Wilkinson S E, Hallam T J. Protein kinase C: is its pivotal role in cellular activation over-stated? Trends Pharmacol Sci 1994; 15(2):53-57; Costello R, Mawas C, Olive D. Differential immuno-suppressive effects of metabolic inhibitors on T-lymphocyte activation. Eur Cytokine Netw 1993; 4(2):139-146), like psoriasis (Sommerer C, Zeier M. AEB071—a promising immunosuppressive agent. Clin Transplant 2009; 23 Suppl 21:15-18; Rasmussen H H, Celis J E. Evidence for an altered protein kinase C(PKC) signaling pathway in psoriasis. J Invest Dermatol 1993; 101(4):560-566; Fisher G J, Tavakkol A, Leach K, Burns D, Basta P, Loomis C, Griffiths C E, Cooper K D, Reynolds N J, Elder J T. Differential expression of protein kinase C isoenzymes in normal and psoriatic adult human skin: reduced expression of protein kinase C-beta II in psoriasis. J Invest Dermatol 1993; 101(4):553-559), rheumatoid arthritis (Healy A M, Izmailova E, Fitzgerald M, Walker R, Hattersley M, Silva M, Siebert E, Terkelsen J, Picarella D, Pickard M D, LeClair B, Chandra S, Jaffee B. PKC-theta-deficient mice are protected from Th1-dependent antigen-induced arthritis. J Immunol 2006; 177(3):1886-1893; Ji J D, Tassiulas I, Park-Min K H, Aydin A, Mecklenbrauker I, Tarakhovsky A, Pricop L, Salmon J E, Ivashkiv L B. Inhibition of interleukin 10 signaling after Fc receptor ligation and during rheumatoid arthritis. J Exp Med 2003; 197(11):1573-1583; Kehlen A, Thiele K, Riemann D, Langner J. Expression, modulation and signalling of IL-17 receptor in fibroblast-like synoviocytes of patients with rheumatoid arthritis. Clin Exp Immunol 2002; 127(3):539-546), or other autoimmune disorders (Zanin-Zhorov A, Dustin M L, Blazar B R. PKC-theta function at the immunological synapse: prospects for therapeutic targeting. Trends Immunol 2011; 32(8):358-363), central nervous system disorders (Liang J, Takeuchi H, Jin S, Noda M, Li H, Doi Y, Kawanokuchi J, Sonobe Y, Mizuno T, Suzumura A. Glutamate induces neurotrophic factor production from microglia via protein kinase C pathway. Brain Res 2010; 1322:8-23; Bastianetto S, Zheng W H, Quirion R. Neuroprotective abilities of resveratrol and other red wine constituents against nitric oxide-related toxicity in cultured hippocampal neurons. Br J Pharmacol 2000; 131(4):711-720), cerebral ischemia or cerebral vasospasm (Bu X, Zhang N, Yang X, Liu Y, Du J, Liang J, Xu Q, Li J. Proteomic analysis of cPKCbetall-interacting proteins involved in HPC-induced neuroprotection against cerebral ischemia of mice. J Neurochem 2011; 117(2):346-356), neuropathies and pain, e.g. neuropathic pain (Nakajima A, Tsuboi Y, Suzuki I, Honda K, Shinoda M, Kondo M, Matsuura S, Shibuta K, Yasuda M, Shimizu N, Iwata K. PKCgamma in Vc and C1/C2 is involved in trigeminal neuropathic pain. J Dent Res 2011; 90(6):777-781; Malmberg A B, Chen C, Tonegawa S, Basbaum A I. Preserved acute pain and reduced neuropathic pain in mice lacking PKCgamma. Science 1997; 278(5336):279-283), cancer development and progression, neoplasia where inhibition of protein kinase C has been shown to inhibit tumor cell growth and metastasis (Kim J, Thorne S H, Sun L, Huang B, Mochly-Rosen D. Sustained inhibition of PKCalpha reduces intravasation and lung seeding during mammary tumor metastasis in an in vivo mouse model. Oncogene 2011; 30(3):323-333; Spindler K L, Lindebjerg J, Lahn M, Kjaer-Frifeldt S, Jakobsen A. Protein kinase C-beta II (PKC-beta II) expression in patients with colorectal cancer. Int J Colorectal Dis 2009; 24(6):641-645; Guo K, Li Y, Kang X, Sun L, Cui J, Gao D, Liu Y. Role of PKCbeta in hepatocellular carcinoma cells migration and invasion in vitro: a potential therapeutic target. Clin Exp Metastasis 2009; 26(3):189-195), angiogenesis (Nakamura S, Chikaraishi Y, Tsuruma K, Shimazawa M, Hara H. Ruboxistaurin, a PKCbeta inhibitor, inhibits retinal neovascularization via suppression of phosphorylation of ERK 1/2  and Akt. Exp Eye Res 2010; 90(1):137-145; Ali A S, Ali S, El-Rayes B F, Philip P A, Sarkar F H. Exploitation of protein kinase C: a useful target for cancer therapy. Cancer Treat Rev 2009; 35(1):1-8; Tekle C, Giovannetti E, Sigmond J, Graff J R, Smid K, Peters G J. Molecular pathways involved in the synergistic interaction of the PKC beta inhibitor enzastaurin with the antifolate pemetrexed in non-small cell lung cancer cells. Br J Cancer 2008; 99(5):750-759; Mischiati C, Melloni E, Corallini F, Milani D, Bergamini C, Vaccarezza M. Potential role of PKC inhibitors in the treatment of hematological malignancies. Curr Pharm Des 2008; 14(21):2075-2084), platelet disorders leading to thrombosis (Gilio K, Harper M T, Cosemans J M, Konopatskaya O, Munnix I C, Prinzen L, Leitges M, Liu Q, Molkentin J D, Heemskerk J W, Poole A W. Functional divergence of platelet protein kinase C(PKC) isoforms in thrombus formation on collagen. J Biol Chem 2010; 285(30):23410-23419; Chari R, Getz T, Nagy B, Jr., Bhavaraju K, Mao Y, Bynagari Y S, Murugappan S, Nakayama K, Kunapuli S P. Protein kinase C[delta] differentially regulates platelet functional responses. Arterioscler Thromb Vasc Biol 2009; 29(5):699-705; Nagy B, Jr., Bhavaraju K, Getz T, Bynagari Y S, Kim S, Kunapuli S P. Impaired activation of platelets lacking protein kinase C-theta isoform. Blood 2009; 113(11):2557-2567; Harper M T, Poole A W. Isoform-specific functions of protein kinase C: the platelet paradigm. Biochem Soc Trans 2007; 35(Pt 5):1005-1008; Strehl A, Munnix I C, Kuijpers M J, van der Meijden P E, Cosemans J M, Feijge M A, Nieswandt B, Heemskerk J W. Dual role of platelet protein kinase C in thrombus formation: stimulation of pro-aggregatory and suppression of procoagulant activity in platelets. J Biol Chem 2007; 282(10):7046-7055; London F S. The protein kinase C inhibitor RO318220 potentiates thrombin-stimulated platelet-supported prothrombinase activity. Blood 2003; 102(7):2472-2481; Wheeler-Jones C P, Patel Y, Kakkar V V, Krishnamurthi S. Translocation of protein kinase C(PKC) in stimulated platelets: a role for aggregation in PKC degradation. Br J Pharmacol 1989; 98 Suppl:845P), and leukocyte aggregation (Hu H, Zhang W, Li N. Glycoprotein IIb/IIIa inhibition attenuates platelet-activating factor-induced platelet activation by reducing protein kinase C activity. J Thromb Haemost 2003; 1(8):1805-1812; Kotovuori A, Pessa-Morikawa T, Kotovuori P, Nortamo P, Gahmberg C G. ICAM-2 and a peptide from its binding domain are efficient activators of leukocyte adhesion and integrin affinity. J Immunol 1999; 162(11):6613-6620; Lorenz H M, Lagoo A S, Hardy K J. The cell and molecular basis of leukocyte common antigen (CD45)-triggered, lymphocyte function-associated antigen-1-/intercellular adhesion molecule-1-dependent, leukocyte adhesion. Blood 1994; 83(7):1862-1870).
Until now, mainly staurosporine derivatives have been described as PKC inhibitors in the prior art, for example Ruboxistaurin (e.g. EP 657458), Enzastaurin (e.g. WO 9517182), Midostaurin (e.g. EP 296110) or Sotrastaurin (e.g. WO 2002038561). Only very few PKCβ inhibitors, which are not derived from staurosporine have been described, such as 3-amido-pyrrolo[3,4-c]pyrazole-5(1H, 4H,6H)carbaldehydes in WO 2008125945. However, there continues to be a need for further effective low molecular weight PKCβ inhibitors, in particular in view of safety and selectivity. The present invention satisfies this need by providing the pyrazolo[3,4-b]pyridine compounds of the formula I.
Pyrazolo[3,4-b]pyridine derivatives which are useful for pharmaceutical applications, have already been disclosed, for example in WO 2005028480 (Neurogen Corp. and Aventis Pharmaceuticals Inc.), in WO 2005009389 (Exelixis Inc.) or in WO 2000058307 (Neurogen Corp.). 6-(4-Hydroxy-phenyl)-1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid amide derivatives are described in EPI 1306228.5 and EPI 1306377.0 (Sanofi). However, there continues to be a need for compounds with an improved profile in terms of metabolic stability and permeability. The present invention satisfies this need by providing the pyrazolo[3,4-b]pyridine compounds of the formula I.